Influences Of Various Biomimetic Coupling Units On The Friction And Wear Behaviors Of Compacted Graphite Cast Iron

Disk brake is one of the most important components in the braking system since its reliability and stability of braking performance will directly influence the traffic safety of railway transportation. As compacted graphite cast iron has high friction coefficient and low wear rate under dry sliding condition, it is one kind of materials that are widely used in vehicle brakes. However, since the development of railway transportation tends to be high-speed and heavy-duty in recent years, the service lives of disk brakes decrease significantly. Moreover, the composite material and ceramic disk brakes have the disadvantages of complicated production technologies, long producing cycles and high manufacturing costs, as well as many pivotal issues needing to be solved, which make them far away from application in railway vehicles. Therefore, it is inevitable and necessary to create a novel kind of disk brakes with high tribological properties, long service lives and low costs. The development history of science indicates that the breakthrough progress has been made in science and technology inspired from the phenomenon of organisms adapting the law of nature. According to some studies concerned, nature provides a whole host of superior multifunctional structures that can be used as inspirational systems for the design and synthesis of new, technologically important materials and devices. Due to the long-term evolution for hundreds of millions of years, the structural characters and morphologies on the surfaces of organisms are optimized. The key factors are materials, morphologies, structures and so on, which have been combined and coupled resulting in high adaptability of organisms to their living environments. Likewise, the biomimetic coupling study for improving the friction and wear behaviors of compacted graphite cast iron has been enlightened by the biological coupling phenomenon in nature, which offers a new way for researching potential materials for disk brakes.In this paper, based on the principle of biomimetic coupling, various biomimetic coupling units mimicking the biological coupling characters on the cuticles of dung beetles, pangolin scales and shells were fabricated on the surfaces of compacted graphite cast iron with different unit materials, dimensions, morphologies and distributions using laser remelting, laser cladding and cast-in process, respectively. The technologies for fabricating different biomimetic coupling units were investigated. The influences of various biomimetic coupling units on the friction and wear behaviors were studies as well. The wear process of biomimetic coupling specimens was analysed. The wear models of different biomimetic coupling units were also presented. Finally, the wear resistance mechanism of compacted graphite cast iron with biomimetic coupling units was discussed. The results are as follows:1. There are two microstructure zones in the biomimetic coupling unit processed by laser remelting, i.e. a melted zone and a transition zone. A fine-grained ledeburite (martensite + cementite) with residual austenite is achieved in the melted zone. The transition zone consists of the martensite and undissolved graphite. Wear tests show that the wear resistance of compacted graphite cast iron is improved by increasing the dimensions of biomimetic coupling units whereas the friction coefficient of the specimens is decreased at the same time. The tribological properties vary with the changes in shapes of biomimetic coupling units. Concerning the shapes of the biomimetic units, the specimen with grid shape biomimetic coupling units exhibits the best wear resistance among the three kinds of the biomimetic coupling specimens; the stria takes the second place and the convex worst with the same laser input energy in the same sliding way. However, the influences of shapes of biomimetic coupling units on the friction coefficient are opposite from the wear resistance results. The specimen with convex units has the highest friction coefficient, the stria takes the second place and the grid is the lowest. The wear mass loss and the friction coefficient are decreased with the decreasing of distributing space between biomimetic coupling units.2. The biomimetic coupling unit fabricated by laser cladding consists of three characteristic microstructure zones, i.e. a cladding zone, a bonding zone and a transition zone. The cladding zone with TiC powders is composed of Fe3C, martensite and TiC, while the cladding zone with WC powders consists of Fe3W3C, Fe3C, WC, W2C, martensite and small amount of graphite. The cladding zone with B4C powders is comprised by FeB, martensite, Fe3C and graphite whereas the cladding zone with Al2O3 powders consists of Fe3C, martensite and Al2O3. The microstructures in bonding zones of specimens cladding different ceramic powders are all ledeburite which is composed by martensite and cementite while those of the transition zones are martensite and undissovled graphite. The wear resistance of specimens with biomimetic coupling units cladding TiC is the best. The specimens cladding WC or B4C powders are in the middle. The specimens cladding Al2O3 is the worst. The sequence of friction coefficient values of specimens with different biomimetic coupling units from high to low is cladding B4C, TiC, WC and Al2O3. The specimens with grid biomimetic units have better wear resistance and higher friction coefficient than those with stria units. With the decrease of spaces between biomimetic units, both the wear resistance and the friction coefficient are improved.3. The biomimetic coupling unit fabricated by cast-in process consists of three characteristic microstructure zones, i.e. an inserted zone, an interface zone and a transition zone. The microstructure of the inserted zone is pearlite. The microstructure of the interface zone is pearlite as well as some amount of secondary cementite. The microstructure of the transition zone is pearlite, secondary cementite and ledeburite. With the increase of biomimetic coupling unit dimensions, the friction coefficient of specimen is increased and the wear resistance is improved. The friction and wear behaviors vary with the changes of unit shapes. The specimens with rectangular and square units have better wear resistance and higher friction coefficient than those of specimens with round units. The friction and wear properties are improved with the decreasing of distributing space between biomimetic coupling units.4. The wear mass losses of biomimetic coupling specimens processed by the three different methods increase with the increase of load, the hardness of counterpart and the ambient temperature. However, the external factors have different influences on the friction coefficient of different biomimetic coupling specimens. As for the biomimetic coupling specimens fabricated by laser remelting, the friction coefficient first increases and then invariant when increasing the load; the friction coefficient increases with the increase of counterpart hardness; and the friction coefficient decreases with the increase of ambient temperature. As for the specimens fabricated by laser cladding, the friction coefficient slightly decreases with the increase of load and the hardness of counterparts. When the ambient temperature is lower than 200oC, the friction coefficient maintains a certain value, afterwards when the ambient temperature is higher than 200oC, the friction coefficient decreases with the increase of ambient temperature.5. The wear mechanism of untreated specimen is mainly adhesion wear, abrasive wear as well as the oxidation wear, whereas the adhesive wear and abrasive wear is the main wear mechanism of the regions of substrate in biomimetic coupling specimens and slight adhesion, abrasive wear, or fatigue wear on the regions of biomimetic coupling units. The wear models are different for biomimetic coupling units processed by different techniques. For the specimens fabricated by laser remelting and cast-in process, adhesion wear is the main mode of the raised particles on the units. But for the specimens processed by laser cladding, fatigue wear is the primary mode of the raised particles on the units.6. There are three stages during the wear process of biomimetic coupling specimens. First, the raised particles on the both sides of frictional pair are removed by the adhesion wear. Since the hardness and wear resistance of biomimetic coupling units are higher than the substrate, the wear extent of substrate is worse than that of units. As a result, biomimetic coupling units project beyond the substrate. Second, the biomimetic units play a dominant role on anti-wear against the counterpart during this stage until the units are worn down to the same level of the substrate. The wear debris accumulates around biomimetic units, which protects the substrate from wear. This period is longer maintains a long time since the strength and hardness of biomimetic units is higher than the substrate. Third, due to the friction heat generated during the wear process, some of the wear debris is oxidized and pressed to form a series of abrasive particles, which scrapes and scratches both of the frictional pair surfaces. As a result, the surface film is broken and new material comes out to the frictional surface again. The substrate is consumed more quickly than the units. Therefore, the units are higher than the substrate again and the above mentioned process will be repeated continuously until the units are completely worn away.7. The reasons of specimens with biomimetic units having better wear resistance than the untreated specimens are as follows: First, the intensified structural effect generated by biomimetic coupling units and wear debris accumulation around the units protect the substrate from wear. Second, the coupling effects of various unit shapes, materials and structures prolong the period of biomimetic units resisting wear, which reduces the wear of the substrate. Third, the friction heat is easier to be transferred since the contact areas and time between the substrate of compacted graphite cast iron and the counterpart are reduced by raised biomimetic coupling units. Finally, the substrate of compacted graphite cast iron can absorb the energy and cushion the impact which makes biomimetic coupling specimens remain the original advantages of compacted graphite cast iron.To sum up, the friction and wear behaviors of compacted graphite cast iron can be improved by biomimetic coupling units, which are coupled by materials, shapes and structures under the biomimetic coupling principle. This method provides a new way to investigate new types of disk brakes. The application of biomimetic coupling technique can not only remain the original advantages of compacted graphite cast iron, such as good vibration reduction and excellent thermal conductance, but also impove the friction and wear behavior by the enhanced biomimetic coupling units on the surfaces of compacted graphite cast iron. Moreover, only a few new fabrication techniques are needed besides the existing materials and equipments. Thus there are many advantages by using this method such as simple technique, low cost and easy operation. Furthermore, the precious resources can be saved since the additions of alloy elements are reduced. Therefore, the study of biomimetic coupling compacted graphite cast iron has offered academic gists and experimental groundwork for the engineering and practical application of biomimetic coupling disk brakes. The development of new type disk brake is of profound theoretical and remarkable economic value.